Progeria is a rare genetic syndrome with many features of premature aging. It may serve as a human genetic disease model of aging. Progeric fibroblast strains may show premature senescence in culture and show a variety of phenotypic abnormalities including decreased growth potential, decreased DNA repair capacity, increased tissue factor activity, and altered cell surface antigens. Cocultivation of progeric cells with normal cells has been shown to stimulate progeric cells to repair DNA. It may be a useful technique to study the reversal of other phenotypic abnormalities. Cocultivation experiments will be used to assess the capacity of normal, senescent, genetic mutant, and other progeric strains to correct or complement cultured progeric phenotypic abnormalities. Genetic heterogeneity will be evaluated by cocultivation. Assays of growth potential, mitotic activity, DNA synthesis, cloning efficiency, DNA repair, and tissue factor activity will be performed. Possible cross-feeding through filters will be examined. DNA ligase activity will be assayed in progeric cells and compared to normals. Karyotype, chromosome, and sister chromatid exchange analysis will be performed.